Userland PHP libraries that depend on this extension may provide higher
level APIs, such as query builders, individual command helper methods, and
GridFS. Application developers should consider using this extension in
conjunction with the
» MongoDB PHP library, which
implements the same higher level APIs found in MongoDB drivers for other
languages. This separation of concerns allows the driver to focus on
essential features for which an extension implementation is paramount for
performance.

"In the new driver, a Cursor object now represents the actual results of an executed command or query, whereas cursors in the legacy driver had a dual-nature (pre- or post-executed). The legacy MongoCursor::count() method actually invoked a count command with the same criteria; however, this meant it was entirely possible for the count command's calculation to differ from the actual query results, since they were not necessarily operating on the same result data.

"The userland library implements a count() method on the collection, which also takes filter criteria, and we'd rather encourage that so users understand that the operations are separate. If we do end up making Cursor countable in the userland library, we would use iterator_count() to provide an exact count on the results; however, this will also require us to cache the results so that users can rewind the cursor and iterate again (this was most recently discussed in PHPLIB-81). While the legacy driver would re-execute the query on a rewind, that's also not an option given the nature of a Cursor in the new driver.

"I realize this may come across as an inconvenience, but I believe the explicit API is a ultimately a step forward from the legacy driver, where many users might have been oblivious to what was actually happening under the hood. Feel free to follow up if you have additional questions.